1. Field of the Invention
The present invention relates to a photographic coupler, and more particularly to a novel yellow color forming coupler which is suitable for use in the formation of color photographic images.
2. Description of the Prior Art
In the formation of color photographic images by a subtractive color reproduction process, an aromatic primary amine compound, especially an N,N-disubstituted para-phenylenediamine compound, is used as a developing agent, to reduce the exposed silver halide grains in an exposed photographic emulsion layer, and the concurrently produced oxidation product of the developing agent is coupled with the color forming coupler to form a cyan, magenta or yellow image dye.
Couplers for use in the color developing process are compounds which have a phenolic hydroxy group, an anilinic amino group or an acitve methylene group, and, by coupling with the oxidation product of the aromatic primary amine developing agent form dyes which absorb light in the visible wave length range.
The yellow dye images exhibit a specific absorption to blue light in the wave length region ranging from about 400 to 500 millimicrons. Previously known yellow color forming couplers include .beta.-ketoacetoacetic esters, .beta.-diketones, N,N-malonic diamides and .alpha.-acylacetamides, and the like.
Of these compounds, benzoyl acetanilide type compounds as described in C. E. K. Mees & T. H. James, The Theory of the Photographic Process, 3rd. Ed., Macmillan, New York (1966) and pivaloyl acetanilide type couplers as described in U.S. Pat. No. 3,265,506 are widely used, as yellow color forming couplers, in the field of color photography. Of these couplers, a large number of the so-called two-equivalent couplers which have a coupling off group at the reactive coupling position thereof are also well known. Examples of two-equivalent couplers having various kinds of coupling off groups are disclosed in U.S. Pat. Nos. 3,447,928; 3,408,194; 3,415,652 and 3,227,155, German Patent Application (OLS) Nos. 2,213,461 and 2,219,917, U.S. Pat. No. 3,253,924, etc. Although these couplers have many preferred properties, they also have a number of disadvantages and are by no means completely satisfactory. For instance, these couplers have poor coupling reactivity, the dyes which are formed from these couplers upon color development with certain useful color developing agents have poor fastness to light, heat or humidity, these couplers have a broad absorption in a longer wave length range, resulting in poor color reproduction, and the coupling off groups leach out from a emulsion layer to a developer solution during color development and adversely affect the properties and storability of the developer solution. Furthermore, the molecular weight of the coupling off groups is large and thus the total molecular weight of the couplers is large, which is disadvantageous in decreasing the thickness of the emulsion layer which is required to improve the sharpness of the image and to shorten the processing time. For example, .alpha.-[4-(4-benzyloxyphenylsulfonyl)-phenoxy]-.alpha.-pivaloyl-2-chloro- 5[.gamma.-(2,4-di-tert-amylphenoxy)butyramidoacetanilide] disclosed in U.S. Pat. No. 3,408,194, .alpha.-(3-benzyl-2,5-dioxo-1-imidazolidinyl)-.alpha.-pivaloyl-2-chloro-5- [.gamma.-(2,4-di-tert-amylphenoxy)butyramido]acetanilide disclosed in Japanese Patent Application (OPI) No. 29432/1973 and .alpha.-(5,5-dimethyl-2,4-oxazolidinedione)-.alpha.-pivaloyl-2-chloro-5[.a lpha.-(2,4-di-tert-amylphenoxy)butyramido]acetanilide disclosed in Japanese Patent Application No. 66835/1974 have good properties but their molecular weights are 908.5, 758.5 and 685.5 respectively, and the weight ratio of the coupling off group in the total molecular weight are 37.3%, 24.9% and 16.9%, respectively. As described above, the weight ratio of the coupling off group which is released upon coupling reaction to form a dye is relatively large with respect to the total molecular weight.